drivers/mma8x5x: correct cast of phydat_t*
drivers/mma8x5x: correct cast of phydat_t*

The read functions have a mma8x5x_data_t* argument which is a
struct with 3 int16_t members. Thus we should cast res->val instead of
just res.

authmillenon/RIOT,ant9000/RIOT,cladmi/RIOT,cladmi/RIOT,basilfx/RIOT,OTAkeys/RIOT,toonst/RIOT,kaspar030/RIOT,mtausig/RIOT,lazytech-org/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,ant9000/RIOT,mtausig/RIOT,toonst/RIOT,yogo1212/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,x3ro/RIOT,rfuentess/RIOT,mtausig/RIOT,miri64/RIOT,x3ro/RIOT,authmillenon/RIOT,aeneby/RIOT,basilfx/RIOT,josephnoir/RIOT,josephnoir/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,BytesGalore/RIOT,x3ro/RIOT,toonst/RIOT,mtausig/RIOT,kaspar030/RIOT,jasonatran/RIOT,mfrey/RIOT,ant9000/RIOT,OTAkeys/RIOT,josephnoir/RIOT,BytesGalore/RIOT,ant9000/RIOT,basilfx/RIOT,mfrey/RIOT,yogo1212/RIOT,smlng/RIOT,x3ro/RIOT,jasonatran/RIOT,aeneby/RIOT,jasonatran/RIOT,A-Paul/RIOT,rfuentess/RIOT,BytesGalore/RIOT,yogo1212/RIOT,lazytech-org/RIOT,A-Paul/RIOT,authmillenon/RIOT,lazytech-org/RIOT,kYc0o/RIOT,A-Paul/RIOT,cladmi/RIOT,toonst/RIOT,cladmi/RIOT,lazytech-org/RIOT,mfrey/RIOT,OlegHahm/RIOT,jasonatran/RIOT,mfrey/RIOT,aeneby/RIOT,yogo1212/RIOT,miri64/RIOT,OTAkeys/RIOT,cladmi/RIOT,mtausig/RIOT,miri64/RIOT,aeneby/RIOT,authmillenon/RIOT,OlegHahm/RIOT,A-Paul/RIOT,x3ro/RIOT,OTAkeys/RIOT,authmillenon/RIOT,yogo1212/RIOT,toonst/RIOT,OlegHahm/RIOT,smlng/RIOT,ant9000/RIOT,basilfx/RIOT,kaspar030/RIOT,smlng/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,A-Paul/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,smlng/RIOT,mfrey/RIOT,rfuentess/RIOT,smlng/RIOT,jasonatran/RIOT,kYc0o/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,BytesGalore/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,basilfx/RIOT,aeneby/RIOT